Telomere length is reflected by plumage coloration and predicts seasonal reproductive success in the barn swallow.

Individuals differ in realized fitness but the genetic/phenotypic traits that underpin such variation are often unknown. Telomere dynamics may be a major source of variation in fitness traits because physiological telomere shortening depends on environmental and genetic factors and may impair individual performance. Here, we showed that, in a population of a socially monogamous, biparental passerine bird, the barn swallow (Hirundo rustica), breeding in northern Italy, telomere length (TL) of both adult males and females positively correlated with seasonal reproductive and fledging success, as expected because long telomeres are supposed to boost performance. Telomere length was correlated with sexually dimorphic coloration in both sexes, showing for the first time in any species that coloration reliably reflects TL and may mediate mutual mate choice, leading to the observed positive assortative mating for TL in the barn swallow. Thus, TL appears to be associated with variation in a major fitness trait and may be an ultimate target of mate choice, as individuals of both sexes can use coloration to adaptively choose high-quality mates that possess long telomeres.